Printing of nanomaterials is not new. The methods available on the market, which are continually being developed, include INKJET (printing using nano-ink), ESJET (electrostatic inkjet printing) and EHD (electrohydrodynamic printing). However, a new technology company from Poland is trying something different, patenting and commercializing a guided assembly of nanoparticles for creating (or, to put it simply, printing) conductive lines with a width of as little as 100 nanometers (nm).
This new comprehensive method for nano-scale printing of conductive lines, characterized by extraordinary flexibility, precision and low cost, has all the features of a so-called disruptive technology. The method is based on a solution to an interdisciplinary scientific and technological problem, including challenges in solid-state physics, inorganic chemistry, nanotechnology, material science, programming, machine learning, mechanics and electronics. The innovative approach is based on a guided assembly of nanoparticles using the dielectrophoretic attraction.
Guided assembly vs printing
“During the process of line formation, the printing head deposits a properly formulated ink – nanoparticles in a mixture of solvents – on a non-conductive substrate, in example such as glass or flexible foil”, says Filip Granek, PhD, the creator of the new method and CEO of XTPL, the company which is commercializing this solution.
An external alternating electric field causes nanoparticles to assemble in a clearly defined and controlled way to form a line. The process takes place between a stationary and movable electrode. The starting point for the process is the stationary electrode, while the printing head performs the function of the movable electrode: it guides the formation of the line, which becomes an extension of the stationary electrode. Finally, the printing head short-circuits with a metallic pad, and the line connects to the pad. When the line formation is complete, the printing head takes in an excess ink.
“You can say that printing in terms of our technology means composing nanoparticles on a substrate,” added Granek.
New quality, new possibilities
“Our solution is often confused with INKJET printing, which is a fundamentally different technology. The INKJET process uses no electrodes, and the voltage is applied only to the piezoelectric printing nozzle. The width of structures printed using INKJET is in the range of 10 to 100 μm, with a limited possibility to further decrease this parameter”, saids Aneta Wiatrowska, PhD, technology director at XTPL.
Likewise, the new solution does not resemble the well-known electrostatic inkjet printing (ESJET) or the electrohydrodynamic (EHD) approach, both of which can be considered competing technologies. The parameters and properties of the final product differ substantially in each process, and thus the possibilities for applying these technologies are different. The method of guided assembly of nanoparticles requires a lower voltage to form a line, i.e., an alternating voltage in the range of ~1 V to ~10 V. On the contrary, the ESJET/EHD process generates pulsed voltage in the range of ~ 100 V to ~1 kV. Using the lower voltage is possible thanks to the movable electrode. This increases the number of potential applications of the technology as it eliminates the risk of damage to the substrate and other components by a very high electric field.
It is worth noting that in the ESJET/EHD process, the electrode is below the substrate, while the solution developed by the Polish scientists puts both electrodes (the stationary and the mobile one) above the substrate.
“With this approach, you can use a substrate of any thickness, including substrates that are not flat”, said Granek.
The feature size of printed structures is one of the key parameters when comparing these technologies. The width of structures obtained using the INKJET method is approximately 10-100 μm, while both the ESJET/EHD methods and the XTPL method allow to print lines thinner than 1 μm. What makes the XTPL technology unique is the possibility to create lines with a width-to-height aspect ratio close to 1 in a single pass of the printing head. In the case of the ESJET/EHD methods, multiple passes are required. Having said that, reducing the width of the printed structures is important as – among others – it increases the transparency of conductive films. In the new method, it is possible to adjust the properties of the conductive lines by changing: the amplitude, shape and frequency of the electrical signal; the physicochemical properties of the ink; and finally the size, distribution and shape of nanoparticles. The latter is another parameter which distinguishes the methods in question from each other. Both in the case of INKJET and ESJET/EHD there are no fundamental limitations on the type of nanoparticles, while the XTPL method may use metallic and semiconductor nanoparticles.
An important feature for each of the analysed methods is the type of substrate on which the structures are printed. In the new method, the printing can be performed on any non-conductive substrate, including ones that are not flat. In the ESJET/EHD methods, thin substrates are preferred to minimise shielding of the electric field. Since fewer restrictions on the substrate extend the range of potential applications, the new technology paves the way for applications in many industry sectors, not accessible to other approaches.
Potential application fields
The solution developed by XTPL will enable the production of a new generation of transparent conductive films (TCF), which are primarily used to manufacture displays, monitors and touch screens. It can be also used for the production of photovoltaic cells, printed electronics, biosensors, lab-on-chip devices and anti-counterfeiting solutions. Particularly promising is the revolutionary application of the technology in the sector of open-defect repair – removal of broken metallic links in thin-film electronic systems. XTPL ‘s research & development team is constantly optimising its innovative technology and adapting the process for further applications.